The purpose of this study is to assess the prophylactic effect of ampicillin on neonatal early-onset infection with group B streptococci (GBS) when administered to women in labor who have colonization with GBS at prenatal visits and major risk factors associated with neonatal infection. The risk factors are premature labor (less than 37 weeks' gestation) and prolonged rupture of amniotic membranes (greater than l2 hours' duration). Screening for GBS is done by vaginal and rectal swabs obtained at a single prenatal visit, usually during the second trimester. During the present grant period, we have demonstrated the validity of a selective approach to chemoprophylaxis based on perinatal risk factors, the predictive value of prenatal cultures with respect to maternal colonization in labor, and the efficacy of selective chemoprophylaxis in interrupting transmission of GBS from mother to infant. Although the data accumulated between September l979 and March l983 indicate a strong trend in favor of the efficacy of selective chemoprophylaxis in preventing bacteremic neonatal disease, we do not expect to reach statistical significance among randomized patients by March l984. Thus, we are requesting on additional year of support to ensure successful completion of this critical aspect of the study.